Electrostatic graphene speaker

ABSTRACT

This disclosure provides systems, methods, and apparatus associated with an electrostatically driven graphene speaker. In one aspect, a device includes a graphene membrane, a first frame on a first side of the graphene membrane, and a second frame on a second side of the graphene membrane. The first frame and the second frame both include substantially circular open regions that define a substantially circular portion of the graphene membrane. A first electrode is proximate the first side of the circular portion of the graphene membrane. A second electrode proximate the second side of the circular portion of the graphene membrane.

RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/US2013/075821, filed Dec. 17, 2013, which claims priority to U.S.Provisional Patent Application No. 61/740,058, filed Dec. 20, 2012, bothof which are herein incorporated by reference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC02-05CH11231 awarded by the U.S. Department of Energy, under GrantNo. N00014-09-1066 awarded by the Office of Naval Research, and underGrant No. EEC-083819 awarded by the National Science Foundation. Thegovernment has certain rights in this invention.

TECHNICAL FIELD

This disclosure relates to devices including graphene, and morespecifically to a graphene speaker.

BACKGROUND

Efficient audio sound transduction has a history dating back millions ofyears. Primitive insect singers generated loud and pure-tone sound withhigh efficiency by exciting resonators inside their bodies. Malecrickets generate chirping sounds via stridulation, where the scraperedge of one wing is rubbed against the ribbed edge of the other wing.Advantageous structural properties of the wings (i.e., relatively large,low-mass flexural membranes) allow efficient muscle-to-sound energytransduction.

In a human context, unnatural (i.e., non-voice) sound production hasbeen explored for millennia, with classic examples being drumheads andwhistles for long-range communications and entertainment. In modernsociety, efficient small-scale audio transduction is ever more importantfor discrete audio earphones and microphones in portable or wirelesselectronic communication devices.

For human audibility, an ideal speaker or earphone should generate aconstant sound pressure level (SPL) from 20 Hz to 20 kHz, i.e., itshould have a flat frequency response. Currently, most commercialspeakers are diaphragm based, and most of the diaphragms are driven by amagnetic coil. Because the coil moves together with the diaphragm, thetotal effective mass becomes large. As a result, the high frequencyresponse may be poor.

To overcome poor high frequency response, acoustic engineers manipulatedamping, which basically decreases the response at lower frequencies tomake the total response curve flat. It is difficult, however, toengineer an arbitrary damping curve at all frequencies. Further, thedamping complicates the acoustic design of the speaker and can increasethe fabrication cost significantly. Another problem created by a largeeffective mass is that it stores kinetic energy which can be releasedlater to jeopardize the music transparency (e.g., the diaphragm does notstart or stop immediately with the input signal).

SUMMARY

Graphene has extremely low mass density and high mechanical strength,key qualities for an efficient wide-frequency-response electrostaticaudio speaker design. Low mass may enable good high frequency response,while high strength may allow for relatively large free-standingdiaphragms necessary for effective low frequency response. Disclosedherein are the fabrication and testing of a miniaturized graphene-basedelectrostatic audio transducer. The speaker/earphone has good frequencyresponse across the entire audio frequency range (20 HZ-20 kHz), withperformance matching or surpassing commercially available audioearphones.

In one aspect, a graphene diaphragm, biased by a DC source, is suspendedmidway between two perforated electrodes driven at opposite polarity. Avarying electrostatic force drives the graphene diaphragm which in turndisturbs air and emits sound through the electrodes. The light mass andlow spring constant of the graphene diaphragm, together with strong airdamping, allow for high-fidelity broad-band frequency response. Such aspeaker also may have a high power efficiency.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a device including a graphene membrane,a first frame on a first side of the graphene membrane, and a secondframe on a second side of the graphene membrane. The first frame and thesecond frame both include substantially circular open regions thatdefine a substantially circular portion of the graphene membrane. Thedevice further includes a first electrode proximate the first side ofthe circular portion of the graphene membrane, and a second electrodeproximate the second side of the circular portion of the graphenemembrane.

In some embodiments, the device further includes a wire in electricalcontact with the graphene membrane. In some embodiments, the wire is agold wire with a diameter of about 10 microns to 30 microns.

In some embodiments, the graphene membrane is about 20 nanometers to 40nanometers thick. In some embodiments, the first electrode and thesecond electrode define open regions having a dimension of about 200microns to 300 microns. In some embodiments, the circular portion of thegraphene membrane has a diameter of about 5 millimeters to 9millimeters. In some embodiments, the first frame and the second frameare about 60 microns to 180 microns thick.

In some embodiments, the first electrode and the second electrodeinclude silicon wafers. In some embodiments, the first electrode and thesecond electrode further include an oxide layer on surfaces of thesilicon wafers.

In some embodiments, the first electrode is in contact with the firstframe and the first electrode is spaced a first distance of about 60microns to 180 microns from the first side of the circular portion ofthe graphene membrane. The second electrode is in contact with thesecond frame, and the second electrode is spaced a second distance ofabout 60 microns to 180 microns from the second side of the circularportion of the graphene membrane.

In some embodiments, the first frame and the second frame are a unitarybody.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a method including provide a device.The device includes a graphene membrane, a first frame on a first sideof the graphene membrane, and a second frame on a second side of thegraphene membrane. The first frame and the second frame both includesubstantially circular open regions that define a substantially circularportion of the graphene membrane. The device further includes a firstelectrode proximate the first side of the circular portion of thegraphene membrane, and a second electrode proximate the second side ofthe circular portion of the graphene membrane. The graphene membrane isbiased with a direct current voltage. The first electrode and the secondelectrode are biased with an input signal, causing the graphene membraneto move and generate a sound.

In some embodiments, the input signal is generated from an audio signal.In some embodiments, the direct current voltage is about 50 volts to 150volts. In some embodiments, an amplitude of the input signal is about 0volts to 15 volts. In some embodiments, the first electrode and thesecond electrode are biased at opposite polarities.

Details of embodiments of the subject matter described in thisspecification are set forth in the accompanying drawings and thedescription below. Note that the relative dimensions of the followingfigures may not be drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example of a cross-sectional schematic illustration ofan electrostatically driven graphene speaker (EDGS).

FIG. 1B shows an example of a graphene membrane suspended in a frame.

FIG. 2 shows an example of a method of use of a device including agraphene membrane.

FIG. 3A-3C show examples of the frequency response of various miniatureaudio speakers.

FIG. 4 shows the vibration velocity of a graphene diaphragm in the EDGSversus frequency, measured by Laser Doppler Velocimetry (LDV).

DETAILED DESCRIPTION

Reference will now be made in detail to some specific examples of theinvention including the best modes contemplated by the inventors forcarrying out the invention. Examples of these specific embodiments areillustrated in the accompanying drawings. While the invention isdescribed in conjunction with these specific embodiments, it will beunderstood that it is not intended to limit the invention to thedescribed embodiments. On the contrary, it is intended to coveralternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention.Particular example embodiments of the present invention may beimplemented without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

Various techniques and mechanisms of the present invention willsometimes be described in singular form for clarity. However, it shouldbe noted that some embodiments include multiple iterations of atechnique or multiple instantiations of a mechanism unless notedotherwise.

Introduction

One approach to response spectrum broadening for speakers is to reduceboth the mass and spring constant of the diaphragm so that inherent airdamping dominates and flattens the response peaks. Moreover, withambient air serving as the dominant damping mechanism, most input energyis converted to a sound wave, which makes such speakers highly powerefficient. For these reasons, the ideal audio transduction diaphragmshould have small mass and a soft spring constant, and be non-perforatedto efficiently displace the surrounding air.

Electrostatically-driven thin-membrane loudspeakers employing anelectrically conducting, low mass diaphragm with significant air dampinghave been under development since the 1920's (e.g., the first were madeof pig intestine covered with gold-leaf), but miniaturized electrostaticearphones are still rare. One reason is that the per-area air dampingcoefficient significantly decreases when the size of the diaphragm fallsbelow the sound wavelength. Hence, for small speakers, a thinner andlower mass density diaphragm is required to continue the dominance ofair damping. Such a diaphragm is difficult to realize. For example, ifconventional materials such as metalized Mylar® are made too thin, theymay fatigue and break.

Graphene is an ideal material for small, efficient, high-qualitybroad-band audio speakers because it satisfies all the above criteria.It is electrically conducting, has a small mass density, and can beconfigured to have small effective spring constant. The effective springconstant of a thin circular membrane isk _(eff)=4πσt  (1)where σ is the stress and t is the thickness of the membrane. It isconvenient to use per-area values for modeling the diaphragm vibrationsince for a given membrane, the mass per unit area is constant. Theequivalent per area spring constant is therefore

$\begin{matrix}{k = {\frac{k_{eff}}{Area} = \frac{4\mspace{2mu}\sigma\; t}{R^{2}}}} & (2)\end{matrix}$where R is the radius of the circular membrane. Note that the springconstant k scales proportionally with the thickness of the membrane andinversely with the 2nd power of the radius of the membrane. Theexceptional mechanical strength of graphene makes it possible toconstruct large and thin suspended diaphragms, which effectively reducesk.

Graphene has been previously used to construct a thermoacousticloudspeaker. In the thermoacoustic configuration, graphene serves as astationary heater to alternately heat the surrounding air, therebyproducing, via thermal expansion, a time-dependent pressure variation(i.e., a sound wave). The method is effective in the ultrasonic regionbecause of graphene's small heat capacity; for this reason, carbonnanotube films can also be utilized. However, for thermoacousticspeakers operating at audio frequencies, most of the input energy isdissipated by heat conduction through the air and does not generatesound. For example, the power efficiency for a graphene thermoacousticspeaker is exceedingly small, decreasing from ˜10⁻⁶ at 20 kHz to ˜10⁻⁸at 3 kHz. The thermoacoustic approach also suffers from sound distortionbecause the heating power is proportional to the square of the inputsignal, and the transduction is therefore intrinsically non-linear.

Apparatus

Described herein are embodiments of an electrostatically driven,high-efficiency, mechanically vibrating graphene diaphragm-based audiospeaker. Even without optimization of the speaker design, a test speakerwas able to produce excellent frequency response across the wholeaudible region (about 20 Hz to 20 KHz), comparable or superior to theperformance of conventional-design commercial counterparts.

FIG. 1A shows an example of a cross-sectional schematic diagram of anelectrostatically driven graphene speaker (EDGS) 100. The EDGS 100includes a graphene membrane 105 suspended in a frame 107 (see FIG. 1Bfor a top-down view) approximately midway between two electrodes 110 and115. In some embodiments, the graphene membrane 105 is a monolayergraphene membrane (i.e., a single layer of graphene). In someembodiments, the graphene membrane 105 is a multilayer graphenemembrane. For example, the graphene membrane 105 may include about 1 to100 layers of graphene. In some embodiments, the graphene membrane 105is about 20 nanometers (nm) to 40 nm thick, or about 30 nm thick.

The frame 107 allows for a portion of the graphene membrane 105 to besuspended or not in contact with other materials. Suspending thegraphene membrane 105 in the frame 107 in this manner may form agraphene diaphragm; a diaphragm is a sheet of a semi-flexible materialanchored at its periphery. In some embodiments, the frame is a disk ofmaterial defining a substantially circular open region, typically in acentral portion of the disk. That is, in some embodiments, the frame issimilar to a hardware washer; a washer is a thin plate of material(typically disk-shaped) with a hole (typically circular and in themiddle) though it. In some embodiments, the frame is about 120 micronsto 360 microns thick, or about 240 microns thick. In some embodiments,the frame has an outer diameter of about 7 millimeters (mm) to 21 mm, orabout 14 mm. The open region defined by the frame may have a diameter ofabout 3 mm to 11 mm, or about 7 mm.

In some embodiments, the frame may include other configurations. Forexample, the frame may define an open region having a rectangular,square, or oval shape, with the material of the frame designed tosuspend the graphene membrane in this open region.

In some embodiments, the graphene membrane 105 is mounted about midwayalong the thickness of the frame 107. For example, when the frame 107 isabout 240 microns thick, the graphene membrane 105 may be mounted to theframe 107 such that about 120 microns of the frame extend from each sideof the graphene membrane. In some embodiments, the graphene membrane isoffset from the midpoint along the thickness of the frame.

In some embodiments, the frame 107 is a polymer, metal, orsemiconducting material. Many different materials could be used for theframe, as long as the material has sufficient mechanical strength tosupport the graphene membrane 105 and to allow for incorporation of theframe 107 into the EDGS 100.

In some embodiments, the frame 107 includes two parts, such that thegraphene membrane 105 is attached to one part of the frame and then theother part of the frame is stacked on top of the graphene membrane,sandwiching the graphene membrane between the two parts of thestructure. For example, a graphene membrane could be suspended in aframe by aligning and attaching two hardware washer shaped parts toeither side of the graphene membrane.

In some embodiments, the graphene membrane 105 is in electrical contactwith a terminal (not shown). In some embodiments, the terminal is ametal wire. For example, in some embodiments, the terminal is a goldwire that is about 10 microns to 30 microns thick, or about 20 micronsthick. In some embodiments, terminals of other materials and of otherdimensions may be used.

The electrodes 110 and 115 are configured to actuate the graphenemembrane 105. In some embodiments, the electrodes 110 and 115 includeperforations 117 so that sound may be emitted from the EDGS 100. Theperforations 117 are though-holes in the electrodes 110 and 115. Theperforations 117 may have any cross-section. For example, in someembodiments, the perforations 117 may have a square cross-section. Insome embodiments, the perforations 117 may have a dimension of about 200microns to 300 microns, or about 250 microns. For example, when theperforations 117 have a square cross-section, the side of a perforationmay be about 200 microns to 300 microns; when the perforations 117 havea circular cross-section, the diameter of a perforation may be about 200microns to 300 microns. In some embodiments, the electrodes are about425 microns to 625 microns thick, or about 525 microns thick.

In some embodiments, one of the electrodes 110 or 115 includesperforations so that sound may be emitted from the EDGS 100. The otherelectrode may define an open region, and not necessarily perforations.The open region may allow the graphene membrane to move; i.e., the openregion may allow for air/gas to enter and exit from between theelectrode and the membrane, which could hinder the movement of themembrane.

The electrodes 110 and 115 may be any material that is capable ofconducting electricity. In some embodiments, the electrodes 110 and 115are silicon electrodes. In some embodiments, an oxide layer 120 isdeposited or formed on the electrodes 110 and 115 to prevent thegraphene membrane 105 from shorting to the electrodes 110 and 115 atlarge drive amplitudes when the EDGS is in operation. In someembodiments, the oxide layer 120 is about 400 nm to 600 nm thick, orabout 500 nm thick. In some embodiments, the oxide layer is a SiO₂layer.

FIG. 2 shows an example of a method of use of a device including agraphene membrane. The method 200 may be similar to elements of theprototype demonstration of an EDGS device described below. Starting atblock 205, a device is provided. The device may be an EDGS, similar to adevice as described above with respect to FIGS. 1A and 1B. At block 210,the graphene membrane of the device is biased with a direct currentvoltage. At block 215, the first electrode and the second electrode ofthe device are biased with an input signal. Biasing the first electrodeand the second electrode may cause the graphene membrane of the deviceto move and generate a sound.

In some embodiments, the input signal is generated from an audio signal.In some embodiments, an amplitude of the input signal is about 0 voltsto 15 volts. In some embodiments, the direct current voltage applied tothe graphene membrane is about 50 volts to 150 volts. In someembodiments, the first electrode and the second electrode are biased atopposite polarities.

EXAMPLES

The following examples are intended to be examples of the embodimentsdisclosed herein, and are not intended to be limiting.

To fabricate an EDGS device, graphene was first synthesized and thenattached to a frame, which was then sandwiched between separatelyfabricated electrodes. Multilayer graphene was synthesized on an about25 micron thick nickel foil in a chemical vapor deposition (CVD) furnaceat about 1000° C. The nickel foil was first annealed at about 1000° C.for about 1 hour with about 50 sccm hydrogen flow at about 200 mTorr,after which the hydrogen flow was increased to about 100 sccm andmethane was introduced at about 5 sccm to start the growth process. Thegrowth pressure was about 2 Torr. After about 20 minutes, the furnacewas turned off and the nickel foil was quickly removed from the hot zoneto allow the formation of graphene layers.

After the growth, a self-adhesive circular frame (e.g., 60 micronsthick) with an about 7 mm diameter opening was attached to the graphenelayer on the nickel foil. The foil was then etched away with an about0.1 g/ml FeCl₃ solution, so that the graphene membrane was only attachedto and supported by the frame. The frame was first transferred to afresh deionized (DI) water bath several times to clean the etchantresidue, and then immersed in acetone. The multilayer graphene diaphragmmay be strong enough to be directly dried in air by pulling the frameout from acetone. In one experimental fabrication process, the thicknessof a free-standing graphene diaphragm was determined by lighttransmission measurement to be about 30 nm thick (e.g., about 22% to 25%transmission) using the above-described fabrication process.

Electrical contact to the graphene membrane was made by attaching anabout 20 micron diameter gold wire to the portion of graphene lying onthe supporting frame. Another circular frame was attached to theoriginal frame from the opposite side (so that the graphene diaphragmwas sandwiched between them) to fix the gold wire. The frames alsoserved as spacers between graphene and the electrodes in the speakerassembly. The gap distance can be increased by stacking multiple (empty)frames on each other. For the EDGS used to generate the resultspresented below, two frames on each side of the graphene were used,which gave a gap distance d of about 120 microns.

The electrodes were constructed from silicon (about 525 microns thick,resistivity of about 10 Ohm·cm, test grade). Photolithography anddeep-reactive-ion-etching operations were used to constructthrough-wafer square holes of about 250 microns wide as sound emittingwindows. An about 500 nm thick protective wet thermal oxide layer wasthen grown on the wafer at 1050° C. The wafer was then diced intoindividual electrodes. Dicing the wafer also exposed the silicon so thatelectric connections are made by attaching conductive wires to the edgesof the electrodes, for example, with silver paste.

To operate the EDGS, the graphene of the EDGS is DC biased at V_(DC).With no input signal, the electrostatic forces from the upper electrodeand the lower electrode balance. When the two electrodes are driven withopposite polarity at V_(in), the total electrostatic force applied ongraphene is (per unit area)

$\begin{matrix}{F = {{F_{1} - F_{2}} = {{{\frac{ɛ}{2\; d^{2}}\left( {V_{DC} + V_{in}} \right)^{2}} - {\frac{ɛ}{2\; d^{2}}\left( {V_{DC} - V_{in}} \right)^{2}}} = {\frac{ɛ\; V_{DC}}{d^{2}}V_{in}}}}} & (3)\end{matrix}$where F₁ and F₂ are force magnitudes due to the respective electrodes, εis the electric permittivity of air, and d is the nominal distancebetween graphene and electrodes. Eq. (3) shows that the actuating forceis linearly proportional to the input signal, an advantage for low sounddistortion.

For prototype demonstration, two electrodes and one graphene diaphragmwere sandwiched together and held by a spring clip. In anotherimplementation, a 7 mm inner-diameter pipe, serving as a wave guide, wasperpendicularly attached to the surface of the electrodes to facilitatesound coupling between the speaker and a listener's ear. A wave guidewill improve far-field efficiency for a small speaker operating atwavelength larger than the diaphragm size.

Described below are prototype demonstration tests for the EDGS.V_(DC)=100 V was used to bias the EDGS device, and the input soundsignal was introduced from a signal generator or from a commerciallaptop or digital music player. The maximum amplitude of the inputsignal V_(in) used in the test was 10 V. The operation current wasusually a few nano-amps, indicating very low power consumption (i.e.,much less than 1 μW) and high power efficiency. In fact, the powerefficiency of an electrostatic speaker can be very high (i.e., closeto 1) because the power dissipation path is almost pure air damping,which converts the mechanical vibration of diaphragm to sound. Magneticcoil type earphones, the type used today for virtually all earphoneapplications, typically have efficiencies less than 0.1.

The sound generated by the graphene speaker was easily audible by thehuman ear. The fidelity was qualitatively excellent when listening tomusic. To quantitatively characterize the speaker, the frequencyresponse curve was measured from 20 Hz to 20 kHz and compared to acommercial earphone of similar size. The sound card in a laptop computerwas used to generate equal-amplitude sine waves, and a commercialcondenser microphone was used to measure the sound pressure level (SPL)at different frequencies.

As can be seen in FIGS. 3A and 3B, the graphene speaker (FIG. 3A), withalmost no specialized acoustic design, performed comparably to a highquality commercial earphone (FIG. 3B). Moreover, the high-frequencyperformance of the EDGS, both in terms of maintaining high response andavoiding sharp resonances (the slow oscillations in the EDGS curve weredue to sound wave interference in the space between the speaker andmicrophone and they depend on the relative position of the speaker andmicrophone, but the main trend is consistent), was markedly better thanthat of the commercial earphone (FIG. 3B) due to the low-mass diaphragm.In the low frequency region, the EDGS and commercial earphone responsecurves both bend downward, likely due to limited capability of thesensing microphone and restricted coupling between the speaker andmicrophone. Nevertheless, the low-frequency performance of the EDGS wasmarkedly better than that predicted for a thermoacoustic speaker (FIG.3C; points in FIG. 3C are experimental data, while the solid line is thetheoretically predicted behavior for an ideal thermoacoustic speaker).At very high frequencies (>10 kHz), the thermoacoustic speakermaintained its good high frequency response, but, as noted above, thepower efficiency was at least six orders of magnitude lower than thatfor the EDGS, which makes it impractical for most portable applications.

The speaker-to-microphone performance test had limited accuracy, becausethe measured response curve is for the whole system—from the sound cardto amplifier, to speaker, to microphone, and finally back to sound card.Every transduction introduces some distortion in the measurement. Forexample, the response was sensitive to the relative position between thespeaker and the microphone. To further demonstrate the capability of theEDGS graphene diaphragm, a laser Doppler velocimetry (LDV) was employedto directly measure the mechanical response limits of the diaphragm.Such a measurement is useful because it eliminates extrinsic effects(e.g., acoustic structural design, sound card, and microphone response),and represents the “pure” response of the graphene diaphragm itself. Themeasured frequency response is illustrated in FIG. 4. Withinexperimental error, the LDV frequency response curve for the EEGSdiaphragm is relatively flat from 20 Hz to 20 kHz, which is the desiredresponse of an ideal speaker diaphragm.

Conclusion

In summary, a robust speaker including a graphene diaphragm (e.g.,single-layer of multi-layer) is disclosed herein. The diaphragm isdriven electrostatically and can reproduce sound with high fidelity.Further, the technique can be scaled to construct larger speakers byarraying the graphene diaphragm.

An EDGS device as described herein could also serve as a microphone. Themicrophone should also have outstanding response characteristics due tothe graphene's low mass and the good coupling to ambient air.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofinvention.

What is claimed is:
 1. A device comprising: a graphene membrane having adiameter of about 5 millimeters to 9 millimeters; a first electrodeproximate a first side of the graphene membrane, the first electrodebeing electrically conductive and having a first oxide layer disposedthereon; and a second electrode proximate a second side of the graphenemembrane, the second electrode being electrically conductive and havinga second oxide layer disposed thereon, the graphene membrane beingsuspended between the first electrode and the second electrode, whereinthe device is a loudspeaker.
 2. The device of clam 1, furthercomprising: a first frame disposed on the first side of the graphenemembrane; and a second frame disposed on the second side of the graphenemembrane, the first frame and the second frame both includingsubstantially circular open regions that define a substantially circularportion of the graphene membrane.
 3. The device of claim 2, wherein thefirst frame and the second frame are about 60 microns to 180 micronsthick.
 4. The device of claim 2, wherein the first electrode is incontact with the first frame, wherein the first electrode is spaced afirst distance of about 60 microns to 180 microns from the first side ofthe circular portion of the graphene membrane, wherein the secondelectrode is in contact with the second frame, and wherein the secondelectrode is spaced a second distance of about 60 microns to 180 micronsfrom the second side of the circular portion of the graphene membrane.5. The device of claim 2, wherein the first frame and the second frameare a unitary body.
 6. The device of claim 1, further comprising: a wirein electrical contact with the graphene membrane.
 7. The device of claim6, wherein the wire is a gold wire with a diameter of about 10 micronsto 30 microns.
 8. The device of claim 1, wherein the graphene membraneis about 20 nanometers to 40 nanometers thick.
 9. The device of claim 1,wherein the first electrode and the second electrode define open regionshaving a dimension of about 200 microns to 300 microns.
 10. The deviceof claim 1, wherein the first electrode and the second electrode includesilicon wafers.
 11. A method comprising: (a) providing a deviceincluding; a graphene membrane; a first electrode proximate a first sideof the graphene membrane, the first electrode being electricallyconductive and having a first oxide layer disposed thereon; and a secondelectrode proximate a second side of the graphene membrane, the secondelectrode being electrically conductive and having a second oxide layerdisposed thereon, the graphene membrane being suspended between thefirst electrode and the second electrode; (b) biasing the graphenemembrane with a direct current voltage; and (c) biasing the firstelectrode and the second electrode with an input signal, causing thegraphene membrane to move and generate an audible sound; wherein thedevice is configured to generate a broad hand response at least from 20Hz to 20 kHz in step (c).
 12. The method of claim 11, wherein the inputsignal is generated from an audio signal.
 13. The method of claim 11,wherein the direct current voltage is about 50 volts to 150 volts. 14.The method of claim 11, wherein are amplitude of the input signal isabout 0 volts to 15 volts.
 15. The method of claim 11, wherein inOperation (c), the first electrode and the second electrode arc biasedat opposite polarities.
 16. The method of claim 11, wherein the devicefurther comprises: a first frame disposed on the first side of thegraphene membrane; and a second frame disposed on the second side of thegraphene membrane, the first frame and the second frame both includingsubstantially circular open regions that define a substantially circularportion of the graphene membrane.
 17. The method of claim 11, whereinthe graphene membrane is about 20 nanometers to 40 nanometers thick. 18.The method of claim 11, wherein the device further comprises: a wire inelectrical contact with the graphene membrane.
 19. The device of claim1, wherein the device is configured to generate a broad band response atleast from 20 Hz to 20 kHz.
 20. The device of claim 1, wherein thedevice is configured to generate a relative sound pressure level of atleast 40 dB at least from 20 Hz to 20 kHz.
 21. The method of claim 11,wherein the device is configured to generate a relative sound pressurelevel of at least 40 dB at least from 20 Hz to 20 kHz in step (c). 22.The device of claim 1, wherein the device is configured to generate auseful response across the entire human audible frequency range.
 23. Themethod of claim 11, wherein the device is configured to generate auseful response across the entire human audible frequency range.
 24. Anelectrostatic audio loudspeaker comprising: a graphene membrane; a firstelectrode proximate a first side of the graphene membrane, the firstelectrode being electrically conductive and haying a first oxide layerdisposed thereon; and a second electrode proximate a second side of thegraphene membrane, the second electrode being electrically conductiveand having a second oxide layer disposed thereon, the graphene membranebeing suspended between the first electrode and the second electrode,wherein the electrostatic audio loudspeaker is configured to generate arelative sound pressure level of at least 40 dB at least from 20 Hz to20 kHz.
 25. The electrostatic audio loudspeaker of claim 24, wherein thegraphene membrane is a monolayer graphene membrane.
 26. Theelectrostatic audio loudspeaker of claim 24, wherein the graphenemembrane is about 20 nanometers to 40 nanometers thick.
 27. Theelectrostatic audio loudspeaker of claim 24, wherein the electrostaticaudio loudspeaker is configured to generate a broad band response atleast from 20 Hz to 20 kHz.
 28. The electrostatic audio loudspeaker ofclaim 24, wherein the electrostatic audio loudspeaker is configured togenerate a useful response across the entire human audible frequencyrange.
 29. The electrostatic audio loudspeaker of claim 24, wherein thegraphene membrane is a multilayer graphene membrane.